3D microscope and methods of measuring patterned substrates

ABSTRACT

A three-dimensional (3D) microscope for patterned substrate measurement can include an objective lens, a reflected illuminator, a transmitted illuminator, a focusing adjustment device, an optical sensor, and a processor. The focusing adjustment device can automatically adjust the objective lens focus at a plurality of Z steps. The optical sensor can be capable of acquiring images at each of these Z steps. The processor can control the reflected illuminator, the transmitted illuminator, the focusing adjustment device, and the optical sensor. The processor can be configured to capture first and second images at multiple Z steps, the first image with the pattern using the reflected illuminator and the second image without the pattern using one of the reflected illuminator and the transmitted illuminator.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application61/367,352, entitled “3D Microscope And Methods Of Measuring PatternedSubstrates”, filed by Hou on Jul. 23, 2010, and incorporated byreference herein. This application is also related to U.S. Pat. No.7,729,049, entitled “3D Optical Microscope”, which issued to Xu on Jun.1, 2010, U.S. Pat. No. 7,944,609, entitled “3D Optical Microscope”,which issued to Xu on May 17, 2011, U.S. Published Application2010/0135573, entitled “3D Optical Microscope” and filed by Xu on Feb.3, 2010, and U.S. Published Application 2008/0291533, entitled“Illuminator For A 3D Optical Microscope” and filed by Xu on Jun. 27,2009, all of which are incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical microscope and in particularto a three-dimensional (3D) microscope and methods of measuring apatterned substrate (PS) in 3D.

2. Description of the Related Art

High Brightness Light Emitting Diode (HBLED) has generated tremendousinterest among research communities and various industries due to itsreliability, long lifetime, and environmental benefits when compared toconventional light sources. Typically, conventional HBLEDs aremanufactured on transparent substrates such as sapphire, siliconcarbide, and other materials. To improve light extraction efficiency,manufacturers often roughen the substrate surface to form patterns sothat a greater portion of light generated in the active layer can beemitted.

U.S. Pat. No. 6,657,236, entitled “Enhanced Light Extraction In LEDsThrough The Use Of Internal And External Optical Element”, which issuedto Thibeault on Dec. 2, 2003, and U.S. Pat. No. 7,384,809, entitled“Method Of Forming Three-Dimensional Features On Light Emitting DiodesFor Improved Light Extraction”, which issued to Donofrio on Jun. 10,2008, disclose methods of creating various repeating patterns on asilicon carbide substrate to enhance the light extraction efficiency ofa HBLED. As described in U.S. Pat. No. 7,384,809, images from asecondary electron microscope (SEM) can be used to verify the shapes ofthese patterned substrates.

U.S. Pat. No. 7,683,386, entitled “Semiconductor Light Emitting DeviceWith Protrusions To Improve External Efficiency And Crystal Growth”,which issued to Tanaka on Mar. 23, 2010, U.S. Pat. No. 7,745,245,entitled “Semiconductor Light Emitting Device”, which issued to Niki onJun. 29, 2010, and U.S. Published Application 2008/0067916, entitled“Light Emitting Device Having A Patterned Substrate And The MethodThereof”, which was filed by Hsu on Jul. 30, 2007, teach various ways togenerate repeating patterns on a sapphire substrate. In thesereferences, SEM images are provided to confirm the quality of thepatterned sapphire substrates.

U.S. Pat. No. 7,704,763, entitled “Highly Efficient Group-III NitrideBased Light Emitting Diodes Via Fabrication Of Features On An N-FaceSurface”, which issued to Fuji on Apr. 27, 2010, discloses a method ofmanufacturing a HBLED on a sapphire substrate, then using laser lift-offto de-bond the substrate from the diode structure. At this point, anetch process can be used to create random pyramids on an N-face GaPsurface to achieve a roughened surface. Again, images from a SEM can beused in monitoring formation of the random pyramid features.

As part of the manufacturing process development and process control,manufacturers need to measure the geometry of the pattern on thesubstrates. These measurements typically include the shape, height,size, pitch, and space of the pattern features. Although a conventionalSEM can image various patterned features, it cannot measure heightinformation. As a result, cross-sectional SEM (x-SEM) has become thestandard metrology tool in the HBLED industry. However, x-SEM is adestructive method, which requires breaking of a HBLED prior to taking ameasurement. In addition, x-SEM measurement has to be carried out in avacuum environment and therefore is slow in throughput. Furthermore, anx-SEM system is expensive to buy and maintain.

Non-destructive, non-contact optical systems have been used in thesemiconductor industry for years in measuring masks on transparentsubstrates. For example, U.S. Pat. No. 6,323,953, entitled “Method AndDevice For Measuring Features On A Transparent Substrate”, which issuedto Blaesing-Bangert on Nov. 27, 2001, and U.S. Pat. No. 6,539,331,entitled “Microscopic Feature Dimension Measurement System”, whichissued to Fiekowsky on Mar. 25, 2003, teach methods for accuratelymeasuring a line width on a photomask using an optical microscope setup.However, these methods can only measure line width, i.e. lateraldimensions, and cannot provide accurate height information.

Therefore, a need arises for a non-destructive method that is accurate,easy to use, and relatively inexpensive to measure and monitor patternedsubstrates. The need is met with the present invention which will beexplained in the following detailed description.

SUMMARY OF THE INVENTION

A three-dimensional (3D) microscope for patterned substrate measurementcan include an objective lens, a reflected illuminator, and atransmitted illuminator. The reflected illuminator can be configured toprovide reflected light for a patterned substrate sample and to projectan image of a patterned article onto and remove the image of thepatterned article from a focal plane of the objective lens. Thetransmitted illuminator can be configured to provide transmittedillumination for the patterned substrate sample.

The 3D microscope can also include a focusing adjustment device, anoptical sensor, and a processor. The focusing adjustment device canautomatically adjust the objective lens focus at a plurality of Z steps.The optical sensor can be capable of acquiring images at each of these Zsteps. The processor can control the reflected illuminator, thetransmitted illuminator, the focusing adjustment device, and the opticalsensor. The processor can be configured to capture first and secondimages at multiple Z steps, the first image with the pattern using thereflected illuminator and the second image without the pattern using oneof the reflected illuminator and the transmitted illuminator.

In one embodiment, the patterned article is a piece of glass with apre-determined pattern thereon. The optical sensor can include acharge-coupled device (CCD) camera or a complementary metal-oxidesemiconductor (CMOS) camera. The transmitted illuminator can be a lightemitting diode (LED) and one of a lens and a lens group. The focusingadjustment device can be a motorized mechanical Z stage or a piezo Zstage. The motorized Z stage can include a lead screw or a ball screwcoupled to a linear bearing. The piezo Z stage can be mounted on asample chuck or a microscope turret.

A method of designing a 3D microscope for measurement of a patternedsubstrate is also described. This method includes providing theabove-described components.

A method of measuring a patterned substrate sample is also described. Apatterned substrate sample is defined as including a plurality ofpatterned substrate features. In this method, a relative distancebetween the patterned substrate sample and an objective lens can bevaried at predetermined steps. At one or more of the predeterminedsteps, the following additional steps can be performed.

An image of a patterned article can be projected onto a focal plane ofthe objective lens. A first image with a pattern associated with thepatterned article and the sample can be captured and then stored in afirst image array. The image of the patterned article can then beremoved from the focal plane of the objective lens. A second image ofthe sample without the pattern associated with the patterned article canbe captured and then stored in a second image array

A first mask can be generated to roughly distinguish the patternedsubstrate features from a background area of the patterned substratesample. This first mask is based on the second image array. A secondmask can be generated to accurately distinguish the patterned substratefeatures from the background area. This second mask is based on thefirst image array and the first mask.

A top of each patterned substrate feature can be determined using thesecond mask and one of the first image array and the second image array.Geometric parameters of patterned substrate features can be calculatedusing the second mask and the top of each patterned substrate feature.

Capturing the second image can include using a reflected illuminator ora transmitted illuminator. The transmitted illuminator can be a lightemitting diode (LED) and one of a lens and a lens group. Generating thefirst mask can include using one of color, intensity, or a combinationof both color and intensity. Generating the second mask can includeusing a thresholding method. The geometric parameters can include size,pitch, height, space, and top size of the patterned substrate features.Varying the relative distance between the patterned substrate sample andthe objective lens can include using a motorized mechanical Z stage or apiezo Z stage. The motorized Z stage can include a lead screw or a ballscrew coupled to a linear bearing. The piezo Z stage can be mounted on asample chuck or a microscope turret.

In one embodiment, the method of measuring a patterned substrate samplecan include automatically varying the relative distance between thepatterned substrate sample and the objective lens. This automaticallyvarying can include a first auto-focus technique and a second auto-focustechnique. The first auto-focus technique can include a conditionalearly exit. This conditional early exit can include determining whethermore than a threshold scan range is done. When more than the thresholdscan range is done, then a standard deviation can be calculated fromaccumulated contrast values, otherwise scanning can continue. Theconditional early exit can further include determining whether themaximum contrast value is more than a specified minimum early exitthreshold and a current step contrast is less than a maximum contrast byat least the standard deviation. If so, then an early exit is approved,otherwise not. The first auto-focus technique can also include capturingimages while the Z stage is moving between scan steps, thereby allowinga speed of the first auto-focus to be as fast as a camera frame rate.

In one embodiment, the second auto-focus technique can have a step sizesmaller than that of the first auto-focus technique. The secondauto-focus technique can include detecting a falling contrast pattern.This falling contrast pattern can be a scan step with peak contrast,followed at least a plurality of scan steps of lower contrast values. Inone embodiment, the plurality of scan steps is four scan steps.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates an exemplary illuminator that can facilitatemeasuring a patterned substrate.

FIG. 1B illustrates an exemplary patterned article.

FIG. 2 illustrates a first embodiment of a 3D microscope systemconfigured to measure patterned substrates.

FIG. 3 illustrates a second embodiment of a 3D microscope systemconfigured to measure patterned substrates.

FIG. 4 illustrates a third embodiment of a 3D microscope systemconfigured to measure patterned substrates.

FIG. 5 illustrates an exemplary light source that can replace the lightsources shown in FIGS. 2 and 3.

FIG. 6 illustrates exemplary positioning components that move the opticsrelative to the sample.

FIG. 7 illustrates exemplary software code and interface usable in theabove-described 3D microscope systems.

FIG. 8 illustrates an exemplary two-pass autofocus technique.

FIG. 9A illustrates an exemplary autofocus first pass technique.

FIG. 9B illustrates an exemplary autofocus first pass early exitdetermination technique.

FIG. 9C illustrates an exemplary autofocus second pass technique.

FIG. 10 illustrates an exemplary patterned substrate measurementtechnique.

FIG. 11 illustrates an exemplary binary mask that can facilitatedistinguishing patterned substrate features from a substrate.

FIG. 12 illustrates an exemplary measurement technique using a manual 3Doptical system.

FIG. 13 illustrates an exemplary measurement technique using anautomatic 3D optical system.

DETAILED DESCRIPTION OF THE DRAWINGS

The term “patterned substrate” as used herein describes a roughenedsurface. This roughened surface can be formed on any transparentsubstrate used in the HBLED industry, e.g. sapphire, silicon carbide,GaP, etc. Embodiments of patterned substrates can use repeating featuresor random features.

FIG. 1A illustrates an exemplary illuminator 100 configured for use in a3D microscope for measuring patterned substrates. Illuminator 100includes two light sources 101 and 102 that can form two light paths(shown as dash-dot lines). Specifically, a first light path includeslight source 101, a first beam-splitter 103, an achromat doublet lens105, a double convex lens 106, and a second beam-splitter 107. A secondlight path includes light source 102, a patterned article 104, firstbeam-splitter 103, achromat doublet 105, double convex lens 106, andsecond beam-splitter 107. A multi-pin connector 108 can activate lightsources 101 and 102 via electrical wires.

In one embodiment, the optical components of illuminator 100 can bemounted inside a dark enclosure with two openings (not shown), e.g. atop opening and a bottom opening. The top opening can be directly abovebeam-splitter 107 while the bottom opening can be directly belowbeam-splitter 107. These two openings allow light from both light pathsto interact with the outside world.

As described in further detail below, after hitting beam splitter 107,the light from one of the first and second sources travels through anobjective lens and then hits the sample surface. Reflected light travelsback through the objective lens, beam splitter 107, and a coupling lens(not shown). A camera receives this reflected light and forms an image(see, e.g. FIG. 2).

In a preferred embodiment, light sources 101 and 102 can include lightemitting diodes (LEDs); however, other light sources such as halogenlamps, fiber-coupled lights, lasers, etc can also be used and are withinthe scope of this invention. Note that although lenses 105 and 106 aredescribed as being an achromat doublet lens and a double-convex lens,those skilled in the art will understand that other types of lenses canalso be used and are within the scope of this invention.

FIG. 1B illustrates one embodiment of patterned article 104. In thisembodiment, patterned article 104 has a surface with a two dimensionalgrid pattern thereon. In other embodiments, different types of patterns,such as an array of evenly spaced opaque dots, can also be used. Indeed,any pattern will work as long as it satisfies the following conditions:(1) it has high contrast, (2) it is either regular or random, (3) it issemi-transparent, and (4) its minimum feature size matches samplingresolution of an imaging optical sensor used.

Note that patterned article 104 can be piece of glass, photographicfilm, or other transparent material that is capable of carrying thepattern. The patterned surface of patterned article 104 is located atthe effective focal plane of the lens group including lenses 105 and106. As described in further detail below, patterned article 104 can beused in illuminator 100 to project an image of the pattern onto thefocal plane of an objective lens to create enough contrast so that 3Dheight information of a sample (e.g. the patterned substrate) can beobtained.

FIG. 2 illustrates a first embodiment of a 3D microscope system 200configured to measure patterned substrates. Note that illuminator 100 isshown in side view in FIG. 2. To avoid unnecessary clutter insideilluminator 100 for illustrating system 200, only light source 101 andbeam splitter 107 are shown. Whenever other components of illuminator100 are mentioned, the reader is advised to reference FIG. 1. Becauseilluminator 100 provides reflected illumination in this configuration,it is called a reflected illuminator. The dash-dot line in FIG. 2illustrates the optical axis along which light travels.

A microscope objective lens 210 is mounted on a turret 260. Turret 260can hold at least one objective lens and is mounted directly below abottom opening of illuminator 100. When light source 101 or 102 isturned on, the lens group including lenses 105 and 106 projects an imageof the light source onto the entrance pupil of microscope objective lens210, thereby ensuring uniform illumination of a sample 220. Moreover,when light source 102 is turned on, the lens group including lenses 105and 106 project an image of the pattern on patterned article 104 ontothe focal plane of objective lens 210.

Positioning means 230 (shown as a double-headed arrow for simplicity) isprovided to change the relative position between sample 220 andobjective lens 210. As a result, different features on sample 220 can bebrought into focus of objective lens 210. In a preferred embodiment,positioning means 230 can include a motorized Z stage or piezo Z stage.In other embodiments, other ways to vary the relative position betweensample 220 and objective lens 210 can be used. For example, objectivelens 210 could be mounted on a piezoelectric actuator, thereby allowingsample 220 to remain stationary while objective lens 210 moves up anddown. Positioning means 230 can also include a manual or motorized XYstage (not shown), thereby allowing sample 220 to be moved in ahorizontal plane. Therefore, positioning means 230 can provide an XYZrange of motion. Those skilled in the art will recognize variations ofthe described positioning means 230.

Coupler 240 in conjunction with objective lens 210 yields an image ofsample 220 on an optical sensor 250. In a preferred embodiment, opticalsensor 250 can be either a charge-coupled device (CCD) or acomplementary metal-oxide-semiconductor (CMOS) camera. Coupler 240 couldbe of a single magnification or of a variable magnification depending onpatterned substrate sample types. For example, coupler 240 could containa 1× lens and a 2× lens mounted on a linear slider.

Light source 280 provides transmitted illumination for sample 220. Assuch, light source 280 is called a transmitted illuminator. In apreferred embodiment, light source 280 is an LED. In other embodiments,light sources such as halogen lamps, fiber coupled lights, lasers, andetc can be used. Sample 220 can sit on a chuck 270, which is formed fromeither a transparent glass plate or a metal plate with a through hole inthe middle to allow light from light source 280 to go through. Aprocessor 290 can be used to control positioning means 230, illuminator100, light source 280, and an optical sensor 250. Processor 290 can alsoanalyze data and create a 3D image of sample 220. In one embodiment,processor 290 can include a personal computer.

FIG. 3 illustrates a second embodiment of a 3D microscope system 300configured to measure patterned substrates. Note that components havingthe same reference numbers (e.g. across various drawings, such as FIGS.2 and 3) indicate that those components provide the same functionalityand therefore are not described in detail again herein. In system 300, alens 301 can be inserted between light source 280 and chuck 270 tobetter concentrate transmitted light. Note that lens 301 can be a singlelens or a group of lenses.

FIG. 4 illustrates a third embodiment of a 3D microscope system 400configured to measure patterned substrates. In this embodiment, twoilluminators 100A and 100B (see, e.g. FIG. 1) can be included in system400 to provide transmitted illumination as well as a means to project animage of patterned article 104 (FIG. 1) onto the focal plane ofobjective lens 210 from the bottom side. A condenser lens 401 can beinserted between light source 101 (in illuminator 100B) and chuck 270 tomatch the numeric aperture of objective lens 210 with that of thetransmitted light.

FIG. 5 illustrates a light source 500 that provides an alternative tolight source 280 used in systems 200 and 300 (FIGS. 2 and 3). In oneembodiment, light source 500 includes an array of light-emitting diodes(LEDs) that can be controlled via an electronics board. The LED arraycan be placed on top of a piezo stage 510. Chuck 270, with either atransparent glass plate or a metal plate with a through hole in themiddle, can be placed on top of light source 500, and sample 220 canthen be placed on top of chuck 270. Note that chuck 270, light source500, and piezo stage 510 are shown spaced apart in FIG. 5 for readercomprehension and in an actual implementation would be secured togetherin a layered configuration.

When the region on sample 220 to be measured is moved under objective210, certain LEDs of the array in proximity to the measured region canbe turned on to provide the transmitted illumination. Piezo stage 510allows for precise vertical movement of sample 220. Note that piezostage 510 provides one possible embodiment of positioning means 230(FIGS. 2 and 4). Further note that sample 220 can be placed on chuck 270with or without light source 500. Positioning means 230 can be used tomove the optics to a nominal focus relative to sample 220. Piezo stage510 can then move sample 220 relative to the optics in higher precisionsteps.

FIG. 6 illustrates another embodiment of positioning means 230 (FIGS. 2,3, 4). In this embodiment, the positioning means can move the opticsrelative to the sample. This movement can be guided by a pair of linearbearings 600. A lead screw or ball screw 610 can be driven by a motor620. To achieve high Z movement resolution, an objective lens can bemounted on a piezo Z drive 630, which in turn can be mounted on anobjective turret. Piezo Z drive 630 can move the objective lens up anddown in accurate steps. Note that the same type of mechanisms can alsobe used to move the sample relative to the optics.

In another embodiment providing high Z movement resolution, a piezo Zdrive can be mounted onto lead screw/ball screw 610. In thisconfiguration, the illuminator, the objective turret, and the objectivelenses can then be moved to a nominal focus position by lead screw/ballscrew 610.

A 3D microscope system can employ two methods of camera control for dataand image acquisition. In a first method, for every scan the systemturns on one of the first and second light sources, moves the Z stage tothe desired position, and issues a trigger signal to the camera toacquire the image. Once image data is transferred from the camera to thecomputer memory, the system switches to the other light source (ifneeded) and issues another trigger signal to the camera. The system thenmoves the Z stage to the next position and repeats the process until thenumber of Z steps is completed.

In a second method, for every scan, the system moves the sample from astarting position to an ending position in a continuous motion withoutstopping. The camera trigger signals are generated from either theposition of the encoder counts of the motor used in the lead screw/ballscrew mechanism, or from the position sensor of the Z-drive or Z-stage.The system electronics then send out the trigger signals at equaldistant intervals to the camera to capture the image. The intervalbetween each trigger is programmed to match with the transfer rate ofthe camera. The system continuously transfers the data to the PC memoryuntil the stage completes its motion. Note that in the second method,the system turns on one of the first and second light sources at thestart of a scan and does not switch light sources during the scan. If asecond pass is required, then the system runs another continuous motionscan using the other light source.

FIG. 7 illustrates exemplary components of 3D microscope system softwareand interface 700 for measuring patterned substrates. An operator caninteract with the 3D microscope system via graphic user interface 710.An auto-focus algorithm 720 can optimize data collection setup andenhance measurement repeatability. A recipe 730 can control dataacquisition parameters and call upon the appropriate analysis algorithm.Patterned substrate analysis package 740 can include various algorithmsto treat the raw data and calculate geometric parameters of a variety ofpatterned substrate features. Reporting package 750 can provideformatted output for patterned substrate feature size, pitch, height,space, etc.

Due to sample thickness variations, different locations on the patternedsubstrate sample may have different Z positions relative to theobjective lens. In addition, a patterned substrate sample is not flatbut has surface texture, i.e. a vertical profile. Therefore, before eachpatterned substrate measurement, the point to be measured on the sampleneeds to be focused. This focusing can be done manually, but theprecision or repeatability of the resulting start point can vary. Tominimize this variation of the start point for a repeatable patternedsubstrate measurement, an auto-focus procedure can be used to startscanning from a consistent starting Z position.

Note that a simple, conventional method to search for the best focuswould be to command the Z stage to step through the whole search rangeat a predefined step size, and at each step, wait until the Z motionsettles, command the camera to capture an image, and then wait for theimage data to arrive. After the image contrast from all steps isanalyzed, the Z position corresponding to the highest contrast could bedetermined. The position of highest contrast would be the best guessfocus Z position. While this simple method works and is accurate, it isundesirably slow.

An auto-focus technique in accordance with the present invention cantake advantage of the 3D microscope image contrast from the projectedpattern as well as from the sample itself. When part of the samplesurface is brought close to the focal plane, the corresponding part ofthe image contrast gets higher and will reach a peak when that part ofthe sample surface is at the focal plane. The auto-focus techniquedescribed herein has two-passes: the first pass being optimized forspeed and the second pass being optimized for accuracy.

FIG. 8 illustrates an exemplary two-pass auto-focus technique 800. Intechnique 800, the positioning means is assumed to be a Z stage, whichcan move the patterned substrate sample up and down. In step 801, thesystem can set up the parameters for the first pass. Exemplaryparameters can include a scan range, a step size and speed, and an earlyexit threshold (described in detail below). In one embodiment, the earlyexit threshold can be user-provided. In step 802, the first pass of theauto-focus is executed, which yields a best guess focus Z position. Instep 803, the offset to the first pass best guess is determined based onempirical results (e.g. previous experiments with typical systems).Table 1 (below) shows typical offsets for various step sizes and cameraframe rates:

TABLE 1 Frame rate (fps) Step size (microns) Offset (microns)  1 to 15up to 0.5 0  1 to 15 more than 0.5 1 16 to 60 up to 0.5 1 16 to 60 morethan 0.5 2 faster than 60 up to 0.5 3 faster than 60 more than 0.5 5

This offset can be used to generate a more accurate first pass bestguess focus Z position (described in further detail in reference to FIG.9A).

In step 804, the Z stage is moved to a second pass starting Z position.In one embodiment, the second pass starting Z position is calculated tobe at half of the second pass scan range below the first pass best guessfocus Z position.

In step 805, parameters for the second pass auto-focus can be set. Forexample, in one embodiment, the step size can be set to be half of thatin the first pass to improve resolution. Moreover, the second pass scanrange can be set to N times the first pass step size, wherein N is apositive integer or fraction. In one embodiment, the scan range is setby a user. In another embodiment, the scan range is set by the recipe,which is specific to a particular sample and system configuration. Anoptimized choice for the second pass scan range may be determined by theaccuracy of the first pass best guess focus Z position. The second passwill be slow if its scan range is set too large. However, if the scanrange is set too small, the actual focus may not be covered, therebypotentially missing the true best focus. In one embodiment, the secondpass scan range may be larger or smaller than 8 times the first passstep size.

In step 806, the second pass auto-focus can be executed to generate asecond pass best guess focus Z position. In step 807, the Z stage can bemoved to a best guess focus Z position plus the final Z offset. Thispositioning can ensure that the patterned sample measurement can startfrom a consistent specific point.

FIG. 9A illustrates exemplary steps for a first pass auto-focus (usablefor step 802). In step 901, the parameters for the first pass auto-focusare accessed. In step 902, the Z stage is moved to a start scanposition. In step 903, an image is captured and the Z stage is commandedto move one scan step. In step 904, the image contrast value iscalculated, then that contrast value and its corresponding Z positioncan be saved in memory.

Notably, in step 903, Z stage movement can be triggered when imageframes are captured rather than arrival at designated scan steps. Thus,images can be captured while the Z stage is moving between scan steps,thereby allowing the auto-focus speed to be as fast as the camera framerate. To speed up each scan step, the first pass auto-focus can run thecamera at its fastest frame rate, which occurs in a free running,continuous capture mode where images are continuously captured andtransferred to processor 290 (FIG. 2). After each image frame iscaptured, the processor commands the Z stage to move to the next scanstep, calculates the image contrast, and then waits for the next imageframe. If the next image frame arrives before the Z stage has completedits motion, the algorithm nevertheless issues a command to move to thenext scan step. Note that because calculation of the contrast value isperformed during movement of the Z stage, steps 903 and 904 are shown inthe same stage in FIG. 9A.

Because the algorithm does not wait for the Z stage to finish itsmotion, the actual Z position corresponding to the image received willbe less than the commanded Z position. This difference is typicallysmall, if the camera frame rate is slow, but becomes more significantwhen the camera is fast. Because of this difference between commandedand actual Z position, the best guess focus Z position corresponding tothe commanded Z position will probably be shifted from the actual bestguess focus Z position. Therefore, a calibration offset table linkingthis shift with the step size and camera frame rate (see, step 803, FIG.8) can be used to compensate for some of this shift, thereby making thefirst pass best guess focus Z position more accurate.

Step 905 determines whether a focus has been found (described in furtherdetail in reference to FIG. 9B), which would allow an early exit. If anearly exit is not possible, then step 906 checks whether the commanded Zposition is equal to a stop scan position. If so, then step 907calculates a first pass best guess focus Z position. The first pass bestguess focus Z position can be the Z position corresponding to the bestcontrast from the captured images of the scan. At this point, step 908can output a first pass best guess focus Z position and stop scanning.If an early exit is possible in step 905, then the first pass techniquecan proceed directly to step 907 and skip step 906. If the commanded Zposition is not equal to a stop scan position, then the first passtechnique can return to step 903 for further scanning.

FIG. 9B illustrates an exemplary technique for a first pass early scanexit determination technique. As noted above, if a valid focus is found,then the first pass can advantageously stop early without scanning therest of the scan range. In general, a focus can be identified if theimage contrast values from the captured images show a pattern of riseand fall. In order to quantify the relative contrast rise and fall, thistechnique requires a minimum number of scan steps to get a meaningfulstatistics calculation of the image contrast. In one embodiment, aminimum of half of the total scan steps is required, but less than halfor more than half is also possible.

Step 921 of this early exit technique can set a default flag of no earlyexit (i.e. early exit is FALSE). Step 922 can determine whether morethan a threshold scan range (i.e. a minimum number of scan steps) isdone. For example, in one embodiment, if less than half of the scansteps are scanned, then step 923 continues scanning and subsequentlyreturns to step 922. If more than half of scan steps are scanned, thenstep 924 can calculate the standard deviation (sigma) from theaccumulated contrast values.

Step 925 can determine whether the maximum contrast value is more than aspecified minimum early exit threshold and the contrast value of currentscan step is at least one standard deviation below that of the maximumcontrast value of the accumulated contrast values. For a typical imagewith a contrast value between 0 and 1000, a threshold of 10 would bereliable for most samples. If so, then a focus is found and step 926 canset the early exit flag to TRUE. If not, then a focus is not found andstep 927 can retain the early exit flag setting of FALSE. Step 928,which follows either step 926 and or step 927, can return to the firstpass technique with an appropriate flag for step 905.

FIG. 9C illustrates an exemplary auto-focus second pass technique. Step931 can access the auto-focus parameters for the second pass and movethe Z stage to its starting position. Step 932 can move the Z stage tothe next scan step and wait until movement is done. Step 933 can capturean image frame at that scan step, calculate its contrast value, and save(in memory) that contrast value as well as its corresponding commanded Zposition.

Because the focus position is expected to be within the second passstarting and ending positions, the saved contrast values in the secondpass are expected to have a rise and fall pattern. Therefore, at step934, a simple check of falling contrast values can be done to determineif focus is found, thereby indicating that the second pass auto-focus isdone. Notably, because the second pass scan step size is smaller thanthat of the first pass, the rise and fall pattern may not be sharp (forexample, several scan steps may have the same or similar maximumcontrast value, thereby not changing contrast values significantly).Also, because the search range of the second pass only needs to coverthe uncertainties of the first pass, it can be much smaller than that ofthe first pass. As a result, the number of search steps in the secondpass steps is small. In the preferred embodiment, the maximum number ofsecond pass search steps is 19. Statistics calculated on such limitednumber of contrast values may not be meaningful. Therefore, in oneembodiment, a falling contrast value can be defined to be a scan stepwith peak contrast, followed by 4 scan steps of lower contrast values. Avalue of less than 4 makes the second pass stop sooner, and a value ofmore than 4 makes the second pass more accurate.

If a falling contrast pattern is detected, then step 935 can calculatethe best guess focus Z position. In one embodiment, the best guess focusZ position can be the middle of the Z position range corresponding tothe middle of the maximum contrast range.

As described above, a two-pass patterned sample measurement techniquecan include first and second passes. The first pass auto-focus canadvantageously stop the focus search early without going through all thesteps. Moreover, the images can be captured while the Z stage is movingbetween scan steps, thereby allowing the auto-focus speed to be as fastas the camera frame rate. To further improve upon the first passautofocus accuracy, the second pass can search at a smaller step sizewithin a small range around the best guess focus Z position found in thefirst pass.

Additional improvements on the two-pass auto-focus technique can also beprovided. For example, in one embodiment, different criteria can be usedfor determining maximum contrast or best focus. That is, instead ofcalculating the overall contrast of the whole image, the contrast of aportion of the image, or the contrast of several different portions ofthe image can be calculated and then used to determine the maximumcontrast for the best focus.

Because the goal of the auto-focus procedure is to position thepatterned substrate sample at a consistent point to start the patternedsample measurement, other means, including different auto-focus methods,such as auto-focus method using image intensity with a confocal opticalsetup, or using focus signal from a separate focus sensor, instead ofimage contrast as described above, can be used in other embodiments toachieve the same results. Such variations are within the scope of thisinvention.

In yet another embodiment, a user can also specify the position foundfrom the auto-focus algorithm to represent the middle, the bottom, orthe top of scan range (or any position in between). This specificity isneeded because the auto-focus algorithm will find the best focusconsistently at the highest average contrast surface. The highestcontrast surface position can be at the base, the middle, or the top ofthe patterned sample structure depending on its shape and composition.This extra control can be specified in the GUI/recipe, thereby tailoringthe GUI/recipe for different patterned sample wafers or chips.

FIG. 10 illustrates an exemplary patterned substrate measurementtechnique 1000. Technique 1000 can receive both patterned image array1004 and non-patterned image array 1001, which includes images capturedat multiple Z positions, as inputs. These inputs can be utilized inmeasuring the size, pitch, height, space, and top size of the patternedsubstrate features. Note that the term “non-patterned image array”refers to an array of images taken without the presence of patternedarticle 104 (see FIG. 1A) in the imaging path. In contrast, the term“patterned image array” refers to an array of images taken with thepresence of patterned article 104 in the imaging path.

For certain patterned substrate features, e.g. such as cone andtriangular features, non-patterned images are collected with thetransmitted illumination (see, e.g. embodiments of FIGS. 2, 3, 4) andthen stored in non-patterned image array 1001. For other patternedsubstrate features, such as flat top features, non-patterned images arecollected with reflected illumination and then stored in non-patternedimage array 1001. In one embodiment, software-implemented binary masks(described below) can be used to roughly and accurately distinguish thepatterned substrate features from the substrate (also referred to as thebackground because the patterned substrate features are above the planeof the substrate) as well as measure the patterned substrate features.Technique 1000 is now explained in detail.

Step 1002 can calculate a histogram spread for non-patterned image array1001 as an indication of either color or intensity distribution. Notethat the spread of the histogram tends to be at the maximum when the Zposition is around the bottom of the patterned substrate features, i.e.at the substrate. Therefore, the Z position of the substrate can beroughly determined by obtaining the maximum of the histogram spread.Step 1003 can generate a first mask by applying a threshold indicated bythe median of the histogram to non-patterned image array 1001 at this Zposition. Note that the first mask is a binary mask having an opaquebackground with transparent features that roughly represent the bases ofthe patterned substrate features. Note that this binary mask isimplemented in software and is not a physical mask.

The first mask can be used to roughly distinguish the patternedsubstrate features from the substrate. The boundary between thepatterned substrate features and the substrate may not be very accuratebecause of noise introduced in part from the actual shape of thepatterned substrate features.

Step 1005 can use patterned image array 1004 and the first mask (fromstep 1003) to calculate the contrast on the substrate at this Zposition. As noted above, the patterned substrate feature designationsof the first mask typically have noise issues. Therefore, step 1005 canuse the first mask to eliminate those areas from the contrastcalculation. As a result, the contrast calculation with patterned imagearray 1004, but without the roughly designated patterned substratefeatures, allows an accurate Z position of the substrate to bedetermined. In one embodiment, this accurate Z position can be based onthe maximum contrast value.

Step 1006 can generate an accurate binary mask, called a second mask, byapplying a threshold indicated by the median of the histogram tonon-patterned image array 1001 at this Z position. This second mask canbe used to accurately distinguish the patterned substrate features fromthe substrate. FIG. 11 illustrates an exemplary second mask 1100, whichshows the bases of the patterned substrate features in white and thesubstrate in black. Note that based on the shape of the white features,the patterned substrate features are cylinders (also called flat tops),domes, or cones.

Step 1007 can use the second (i.e. accurate) mask and either thenon-patterned image array 1001 or the patterned image array 1004 tocalculate the top and the Z position of each of the patterned substratefeatures. Information from the recipe (described below in reference toFIG. 12) regarding the patterned substrate feature shape can be used instep 1007 to determine whether to use non-patterned image array 1001 orpatterned image array 1004. Specifically, if the shape of the patternedsubstrate features is conical or triangular, then step 1007 can usenon-patterned image array 1001. However, if the shape of the patternedsubstrate features is flat (e.g. cylindrical), then step 1007 can usepatterned image array 1004. In one embodiment, interpolation can be usedto take into account possible variations and/or combinations in patternsubstrate feature shapes. The calculations performed in step 1007 can bebased on contrast computations for pixels inside of each feature.

Step 1008 can calculate the size, pitch, height, space, and top size ofeach patterned substrate feature based on the top and Z position valuescomputed in step 1007 and the second mask. Size can be defined as adiameter for a circular patterned substrate feature or a height for apatterned substrate having an equilateral triangular base. Pitch can bedefined as a distance between the center of a current patternedsubstrate feature and the center of a neighboring patterned substratefeatures. In one embodiment, the average of the distances between allneighboring patterned substrate features can be calculated and used asthe pitch. Height can be determined by the absolute difference of themaximum Z position and the minimum Z position within the patternedsubstrate feature. Space can be defined as the difference between thepitch and the size. Note that the top size only applies to a patternedsubstrate feature having a flat plane on its top. Statistics of thesevalues, such as average, median, standard deviation and others can beobtained over the whole field of view to get more reliable readings ofthe patterned substrate features. Step 1009 can report these values.

FIG. 12 illustrates exemplary steps in a patterned substrate measurementtechnique 1200 using a manual 3D microscope system in accordance withone embodiment. Note that a manual system is defined as one having amanual XY stage. At step 1201, an operator can load a patternedsubstrate sample, choose an objective lens, and locate a measurementspot on the sample. For patterned substrate measurement, an objectivelens (i.e. objective lens 210, FIG. 2) having a 100× magnification lenswith 0.9 or 0.95 numerical aperture can be chosen. In step 1202, theoperator can load a recipe corresponding to the sample. In step 1203,the operator can initiate data acquisition by clicking on a button. Atthat point, the system can get and analyze raw data to provide thenecessary output information. In step 1204, the operator can save theresults and/or conduct specific, selected analysis. In step 1205, theoperator can decide whether more spots on the sample are to be measured.If so, then the technique can return to step 1203. If not, thentechnique 1200 can proceed to step 1206, at which point the operator canunload the sample and the patterned sample measurement technique ends.

FIG. 13 illustrates exemplary steps in a patterned substrate measurementtechnique using an automated 3D microscope system in accordance withanother embodiment. An automated system is defined as one with amotorized XY stage. In step 1301, an operator can load a patternedsubstrate sample and choose an objective lens. For patterned substratemeasurement, an objective lens having a 100× magnification lens with 0.9or 0.95 numerical aperture typically is chosen. In step 1302, theoperator can load a sequence file that includes, among other things, arecipe corresponding to the sample and a map of predeterminedmeasurement locations. In step 1303, the operator can initiate dataacquisition, e.g. by clicking a button. At that point, the system willmove to the first measurement spot on the sample, take raw data, analyzethe raw data to provide the necessary result, and save the results intoa file. When the measurement is done, the sample is automatically movedto the next spot. This process will be repeated until all the designatedlocations are measured and the results are saved. At step 1304, theoperator can unload the sample, thereby terminating the patterned samplemeasurement technique.

Compared to prior art, the 3D microscope system described herein hasseveral advantages. Specifically, the 3D microscope system is easy touse, is based on a non-contact, non-destructive method, offers a lowcost of ownership among a class of existing patterned sample measurementtools, notably, provides fast, accurate, and repeatable measurement onkey parameters that matter to patterned substrate manufacturers. Amongthese parameters are the size, pitch, height, and space of patternedsubstrate features.

The embodiments described herein are not intended to be exhaustive or tolimit the invention to the precise forms disclosed. As such, manymodifications and variations will be apparent. Accordingly, it isintended that the scope of the invention be defined by the followingClaims and their equivalents.

The invention claimed is:
 1. A method of measuring a patterned substratesample, the patterned substrate sample including a plurality ofpatterned substrate features, the method comprising: varying a relativedistance between the patterned substrate sample and an objective lens atpredetermined steps; at one or more of the predetermined steps:projecting an image of a patterned article onto a focal plane of theobjective lens; capturing a first image with a pattern associated withthe patterned article and the sample, and storing the first image in afirst image array; and capturing a second image of the sample withoutthe pattern associated with the patterned article, and storing thesecond image in a second image array; roughly determining a Z positionof a background area of the patterned substrate sample using the secondimage array, and generating a first mask to roughly distinguish thepatterned substrate features from the background area of the patternedsubstrate sample based on the roughly determined Z position; generatinga second mask to accurately distinguish the patterned substrate featuresfrom the background area based on the first image array and the firstmask; determining a top of each patterned substrate feature using thesecond mask and one of the first image array and the second image array;and calculating geometric parameters of patterned substrate featuresusing the second mask and the top of each patterned substrate feature.2. The method of claim 1, wherein capturing the second image includesusing one of a reflected illuminator and a transmitted illuminator. 3.The method of claim 2, wherein the transmitted illuminator is a lightemitting diode (LED) and one of a lens and a lens group.
 4. The methodof claim 1, wherein generating the first mask includes using one ofcolor, intensity, or a combination of both color and intensity.
 5. Themethod of claim 1, wherein generating the second mask includes using athresholding method.
 6. The method of claim 1, wherein the geometricparameters include size, pitch, height, space, and top size of thepatterned substrate features.
 7. The method of claim 1, wherein varyingthe relative distance between the patterned substrate sample and theobjective lens includes using one of a motorized mechanical Z stage anda piezo Z stage.
 8. The method of claim 7, wherein the motorized Z stageincludes one of a lead screw and a ball screw coupled to a linearbearing.
 9. The method of claim 7, wherein the piezo Z stage is mountedon one of a sample chuck and a microscope turret.
 10. A method ofmeasuring a patterned substrate sample, the patterned substrate sampleincluding a plurality of patterned substrate features, the methodcomprising: automatically varying a relative distance between thepatterned substrate sample and an objective lens at pre-determinedsteps, the automatically varying including an auto-focus; at one or moreof the pre-determined steps: projecting an image of a patterned articleonto a focal plane of the objective lens; capturing a first image with apattern associated with the patterned article and the sample, andstoring the first image in a first image array; and capturing a secondimage of the sample without the pattern associated with the patternedarticle, and storing the second image in a second image array; roughlydetermining a Z position of a background area of the patterned substratesample using the second image array, and generating a first mask toroughly distinguish the patterned substrate features from the backgroundarea of the patterned substrate sample based on the roughly determined Zposition; generating a second mask to accurately distinguish thepatterned substrate features from the background area based on the firstimage array and the first mask; determining a top of each patternedsubstrate feature using the second mask and one of the first image arrayand the second image array; and calculating geometric parameters ofpatterned substrate features using the second mask and the top of eachpatterned substrate feature.
 11. The method of claim 10, whereincapturing the second image includes using one of a reflected illuminatorand a transmitted illuminator.
 12. The method of claim 11, wherein thetransmitted illuminator is a light emitting diode (LED) and one of alens and a lens group.
 13. The method of claim 10, wherein generatingthe first mask includes using one of color, intensity, and a combinationof both color and intensity.
 14. The method of claim 10, whereingenerating the second mask includes using a threshold method.
 15. Themethod of claim 10, wherein the geometric parameters include size,pitch, height, space, and top size of the patterned substrate features.16. The method of claim 10, wherein varying the relative distancebetween the patterned substrate sample and the objective lens includesusing one of a motorized mechanical Z stage and a piezo Z stage.
 17. Themethod of claim 16, wherein the motorized Z stage includes one of a leadscrew and a ball screw coupled to a linear bearing.
 18. The method ofclaim 16, wherein the piezo Z stage is mounted on one of a sample chuckand a microscope turret.
 19. The method of claim 10, wherein theautofocus includes a first auto-focus technique and a second autofocustechnique.
 20. The method of claim 19, wherein the first auto-focustechnique includes a conditional early exit.
 21. The method of claim 20,wherein the conditional early exit includes determining whether morethan a threshold scan range is done.
 22. The method of claim 21, whereinwhen more than the threshold scan range is done, then calculating astandard deviation from accumulated contrast values, otherwise continuescanning.
 23. The method of claim 22, wherein the conditional early exitfurther includes determining whether the maximum contrast value is morethan a specified minimum early exit threshold and a current stepcontrast is less than a maximum contrast by at least the standarddeviation.
 24. The method of claim 20, wherein the first auto-focustechnique includes capturing images while the Z stage is moving betweenscan steps, thereby allowing a speed of the first auto-focus to be asfast as a camera frame rate.
 25. The method of claim 20, wherein thesecond auto-focus technique has a step size smaller than that of thefirst auto-focus technique.
 26. The method of claim 25, wherein thesecond auto-focus technique includes detecting a falling contrastpattern.
 27. The method of claim 26, wherein the falling contrastpattern is a scan step with peak contrast, followed at least a pluralityof scan steps of lower contrast values.
 28. The method of claim 27,wherein the plurality of scan steps is four scan steps.